1. Field of the Invention
The present invention relates to a display device, and more particularly to an organic EL display device of a type which forms organic EL layers using a transfer method, and a manufacturing method of the display device.
2. Background Art
In an organic EL display device, an organic EL layer is sandwiched between a lower electrode and an upper electrode, a fixed voltage is applied to the upper electrode and a data signal voltage is applied to the lower electrode so as to control emission of light from the organic EL layer thus forming an image. The data signal voltage is supplied to the lower electrode via a thin film transistor (TFT).
An organic EL display device is classified into a bottom-emission-type organic EL display device in which light emitted from organic EL layers is taken out in the direction of a glass substrate on which the organic EL layers and the like are formed, and a top-emission-type organic EL display device in which light emitted from organic EL layers is taken out in the direction opposite to a glass substrate on which the organic EL layers and the like are formed. The top-emission-type organic EL display device has an advantage that each organic EL layer can ensure a light emission area also on a TFT or the like thus acquiring large brightness.
The organic EL layer is formed of a plurality of layers, and a thickness of each layer is extremely small. Further, properties of the organic EL layer are deteriorated by moisture and hence, wet etching cannot be used for forming the organic EL layer. Accordingly, conventionally, the organic EL layer is formed by a vacuum vapor deposition method. Further, for forming a color image, the organic EL layers are formed by using materials for forming light emitting layers which differ in color by vapor deposition using a mask. However, in view of the accuracy of the mask per se, the alignment accuracy between the mask and a substrate, the thermal expansion difference between the mask and the substrate or the like, the vapor-deposition position accuracy is insufficient so that the manufacture of a display with high accuracy has been difficult.
Large-sizing of a substrate is extremely effective for enhancing the production efficiency. However, a high-definition mask corresponding to the large-sized substrate increases a weight thereof due to a frame which imparts a tension to the mask for preventing sagging of the mask and hence, handling of the mask is difficult. In spite of an expensive unit price of the mask, when continuous vapor deposition is performed using one mask, films are stacked so that mask accuracy is lost whereby it is necessary to perform cleaning each time a cumulative film thickness reaches several μm. In the actual mass production of the organic EL display devices, it is necessary to prepare a considerably large number of masks. A high vacuum device of 10−4 Pa is necessary as a vacuum vapor deposition device so that the vacuum vapor deposition device is expensive as a mass production facility. In addition to these drawbacks, in the vapor deposition, only several % of an evaporated material is deposited on a surface of the substrate leading to a wasteful use of an expensive OLED material.
As a method which can obviate these drawbacks, a laser transfer method has been proposed. In the laser transfer method, however, laser beams are radiated to a transfer substrate from a backside and a film pattern is formed on a surface opposite to the back surface and hence, the whole pattern is drawn by laser beams on the large-sized substrate thus giving rise to a drawback that a manufacturing time is prolonged. Further, considerably high pattern drawing accuracy is requested so that a stage formed with high accuracy is necessary and hence, a cost for manufacturing the device is not largely lowered although the vapor deposition is not adopted. As documents which describe such a transfer method, JP-A-9-167684 (patent document 1) and JP-A-2006-309994 (patent document 2) are named.